This invention relates to improved methods and compositions for investment casting technology.
Investment casting by the lost wax process can be traced to ancient Egypt and China. The process as practiced today, however, is a relatively new technology dating to the 1930""s and represents a rapidly growing business and science. Investment casting technology simplifies production of complex metal shapes by casting molten metal into expendable ceramic shell molds formed around disposable wax patterns which duplicate the desired metal shape. xe2x80x9cPrecision Investment Castingxe2x80x9d, i.e., PIC, is the term in the art that refers to this technology.
The Conventional PIC Process Employs Six Major Steps:
(1) Pattern Preparation.
A disposable positive pattern of the desired metal casting is made from a thermoplastic material such as wax that will melt, vaporize or burn completely so as not to leave contaminating residues in the de-waxed ceramic shell mold. The positive pattern is prepared by injecting the thermoplastic material into a negative, segmented, metal die or xe2x80x9ctoolxe2x80x9d designed to produce patterns of the shape, dimension and surface finish required for the metal casting. Single or multiple patterns can be assembled by fusing them to a disposable wax xe2x80x9csprue systemxe2x80x9d that feeds molten metal to fill the shell mold;
(2) Shell Mold Construction By:
(a) dipping the pattern assembly into a refractory slurry having fine particulate refractory grain in an aqueous solution of alkali stabilized colloidal silica binder to define a coating of refractory material on the pattern;
(b) contacting the refractory coating with coarse dry particulate refractory grain or xe2x80x9cstuccoxe2x80x9d to define a stucco coating, and
(c) air drying to define a xe2x80x9cgreenxe2x80x9d air dried insoluble bonded coating. These process steps can be repeated to build by successive coats a xe2x80x9cgreenxe2x80x9d, air dried shell mold of the desired thickness.
(3) Dewaxingxe2x80x94The disposable wax pattern is removed from the xe2x80x9cgreenxe2x80x9d air dried shell mold by steam autoclaving, plunging the green shell mold into a flash de-waxing furnace heated to 1000xc2x0 F.-1900xc2x0 F., or by any other method which rapidly heats and liquefies the wax so that excessive pressure build-up does not crack the shell mold.
(4) Furnacingxe2x80x94The de-waxed shell mold is heated at about 1600xc2x0 F.-2000xc2x0 F. to remove volatile residues and form stable ceramic bonds in the shell mold.
(5) Pouringxe2x80x94The heated shell mold is recovered from the furnace and positioned to receive molten metal. The metal may be melted by gas, indirect arc, or induction heating. The molten metal may be cast in air or in a vacuum chamber. The molten metal may be poured statically or centrifugally, and from a ladle or a direct melting crucible. The molten metal is cooled to produce a solidified metal casting in the mold.
(6) Casting recoveryxe2x80x94The shell molds having solidified metal castings therein are broken apart and the metal castings are separated from the ceramic shell material. The castings can be separated from the sprue system by sawing or cutting with abrasive disks. The castings can be cleaned by tumbling, shot or grit blasting.
Investment casting shell molds tend to be fragile and prone to breakage. In an effort to improve the strength of investment casting shell molds, small amounts of chopped refractory fibers and chopped organic fibers have been added to aqueous refractory slurries. Refractory slurries which include these have small amounts of chopped refractory fibers enable application of thicker coatings to a preform. These slurries, however, require significant amounts of polymer to achieve satisfactory green strength and flow properties of the slurry.
A need therefore exists for materials and methods which provide investment casting shell molds which have improved strength and avoids the disadvantages of the prior art.
The invention relates to a process for rapidly forming a ceramic shell mold on a disposable support member, and to the ceramic shell molds obtained thereby. The process entails forming a dry blend that includes refractory fiber, glass fiber and refractory filler. The dry blend then is mixed with an aqueous colloidal silica sol binder to form a refractory slurry. The refractory slurry then can be used in the manufacture of ceramic shell molds.
The invention relates to a method of manufacture of an investment casting mold. The method entails mixing refractory fiber, glass fiber, and refractory filler to form first dry blend; mixing the first dry blend with an aqueous colloidal silica sol to form a refractory prime coat slurry; mixing refractory filler, glass fiber and refractory fiber to form a second dry blend, mixing the second dry blend with an aqueous colloidal silica sol to form a refractory back-up coat slurry which may be the same or different from the refractory prime coat slurry; applying a coating of the prime coat slurry onto an expendable pattern to produce a prime coated preform; applying a stucco of refractory material onto the prime coated preform; drying the stuccoed, prime coated preform; applying a coating of refractory back-up coat slurry onto the stuccoed, prime coated preform to produce a refractory back-up coated preform; applying a stucco of refractory material onto the back-up coated preform to produce a stuccoed, back-up coated preform; drying the stuccoed, refractory back-up coated preform; removing the expendable pattern from the refractory back-up coated preform to produce a green shell mold; and heating the green shell mold to a temperature sufficient to produce a ceramic shell mold. The refractory fiber can be a ceramic fiber and the refractory filler can include ceramic grains. The ceramic fiber can be present in an amount of about 1 wt. % to about 10 wt. % by weight of the dry blend, the glass fiber can be preseent in an amount of about 0.5 wt. % to about 10 wt. % by weight of the dry blend, and the refractory filler can be present in an amount of about 80 wt. % to about 98.5 wt. % by weight of the dry blend. The dry blend can further include a polymeric fiber.
Where the dry blend includes a polymeric fiber, the ceramic fiber can be present in an amount of about 1 wt. % to about 10 wt. % by weight of the dry blend, the glass fiber can be present in an amount of about 0.5 wt. % to about 10 wt. % by weight of the dry blend, and the refractory filler can be present in an amount of about 80 wt. % to about 98.5 wt. % by weight of the dry blend, and the polymeric fiber can be present in an amount of about 0.3 wt. % to about 4 wt. % by weight of the dry blend.
The process of the invention offers a number of advantages for the manufacture of ceramic shell molds over the prior art. For example, forming dry blends of fibers and refractory filler enables easy addition of refractory filler and fibers to the colloidal silica sol binder without the need to continuously mix or re-mix the silica sol and fiber pre-blend prior to use. Another advantage is that the fibers do not need to be pre-dispersed in a liquid binder or combined with a polymer prior to adding refractory filler. A further advantage is that polymeric binder additives are not required to achieve improved green strength. Other advantages are that the invention avoids the prior art problem of fiber agglomeration under high shear mixing and the ability to build thicker coatings per dip.